The present invention relates to a multiplex transmission system using a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) transmission system, and more particularly, to a frame format used in the multiplex transmission system.
There has been proposed a distributed network type multiplex transmission system using a CSMA/CD transmission system, wherein a plurality of multiplex nodes are coupled together through a multiplex transmission path, data is transmitted in units of frame each including a destination address from any one of the nodes, and when the node designated by the destination address receives the data properly, it returns a reception acknowledgment signal upon receiving the frame properly. FIG. 1 schematically shows the structure of a multiplex transmission system for vehicles which uses the CSMA/CD transmission system. As illustrated, a plurality of multiplex nodes comprising, for example, a front multiplex node FN, a combination switch multiplex node CS, a meter multiplex node MT, and a rear multiplex node RN, are coupled together through a multiplex transmission path (bus) MB composed, for example, of a twisted pair type wire.
The front multiplex node FN is coupled to a front turn-right signal lamp 6, a front turn-left signal lamp 7, a front small lamp 8, and a horn 9. The combination switch multiplex node CS is coupled to a turn-right switch 10, a turn-left switch 11, a small lamp switch 12, a horn switch 13, and a head-lamp high beam switch 14. The meter multiplex node MT is coupled to a turn-right indicator 15, a turn-left indicator 16, and a head-lamp high beam indicator 17. The rear multiplex node RN is coupled to a rear turn-right signal lamp 18, a rear turn-left signal lamp 19, and a tail lamp 20 (which is turned on when the small lamp switch 12 is on).
In this multiplex transmission system for vehicles, vehicle driving information is transmitted in units of frame F each having the format as shown in FIG. 2. The frame F includes an SD (Starting Delimiter) code, a priority code, a frame ID code, data, and a check code.
The SD code is a specific code indicating the start of the frame F. The reception-side multiplex node recognizes the start of the frame F upon receiving the SD code. The priority code is a code for priority control, which serves to indicate the priority of signals to be transmitted when a plurality of multiplex nodes concurrently transmit data and accordingly the signals therefrom collide with one another. When a plurality of data collide with one another, that with greater priority is transmitted first. The frame ID code is a code for indicating the type of the frame with which it is associated. More specifically, it indicates what data is assigned to a respective bit of the data area, that is, what combination of data is contained in the data area. The reception multiplex node recognizes the content of the data in the data area of the transmitted frame by means of the frame ID code. In the data area is written, bit by bit, the on/off data of the head lamp, turn-right lamp, turn-left lamp, horn, small lamp, head-lamp high beam switch, head-lamp low beam switch, etc. The check code (error detection code) is transmitted following the data. The reception multiplex node recognizes the end of the frame by detecting this code. Moreover, in order to ensure the data security, each reception node judges whether or not the content of the received frame is correct by means of the check code, and if the transmitted data is correct, the reception node sends its local address, as a reception acknowledgment signal (ACK signal), onto the transmission path MB upon passage of a certain time within a predetermined period of time. The multiplex node which has transmitted the frame F receives the ACK signals, so that it recognizes that the receiving side properly received the data. When the data is not properly received by the receiving side (a data error is detected by means of the check code), or a framing error occurs (the data actually transmitted is shorter or longer than is designated by the data length), the receiving side returns no ACK signal. If the transmission side does not receive the expected ACK signal within the predetermined period of time after the frame transmission, it starts retransmitting the frame F.
Motor vehicles include a variety of models and, moreover, even vehicles of the same model can be classified into several grades or classes. Thus, the multiplex transmission systems for vehicles usually differ in system configuration from one model to another. Consequently, the assignment of data in the transmission frame varies depending on the model and grade of vehicles in which the frame is to be used. For example, as shown in FIG. 3, a vehicle of model A has head-lamp switches and turn signal lamp switches located adjacent to one another. In this model, therefore, data on these switches can be input collectively to a multiplex node if the node is situated near the switches. Therefore, the combination switch multiplex node CS is provided with the turn-right switch 10, turn-left switch 11, head-lamp high beam (Hi) switch 14, and head-lamp low beam (Lo) switch 14', and the front multiplex node FN is situated on one side of the vehicle so as to be close to the left- or right-side lamps. The on/off signals of the front turn signal lamps 6 and 7 and the high- or low-beam signal of the head lamps 22 are transmitted from the combination switch multiplex node CS to the front multiplex node FN in accordance with the frame format shown in FIG. 5(a). In the example of FIG. 5(a), the head-lamp high beam signal, head-lamp low beam signal, turn-right signal and turn-left signal are assigned respectively to the zeroth, first, second and third bits in the data area of a single frame. Upon receiving this frame, the front multiplex node FN causes the front turn signal lamps 6 and 7 and the head lamps 22 to operate in accordance with the frame data.
On the other hand, in the case of another vehicle model B, it is desirable to separate the signal of the head lamps from the signals of the turn signal lamps, with their corresponding switches connected to different multiplex nodes, in view of the operation of the multiplex transmission system. Therefore, as shown in FIG. 4, the high beam switch 14 and the low beam switch 14' of the head lamps 22 are provided in the combination switch multiplex node CS, while the turn-right switch 10 and the turn-left switch 11 are provided in a cluster switch multiplex node CN arranged separately. Thus, the high- or low-beam signal of the head lamps 22 is transmitted from the combination switch multiplex node CS to the front multiplex node FN in accordance with the frame format shown in FIG. 5(b), and the on/off signals of the front turn signal lamps 6 and 7 are separately transmitted from the cluster switch multiplex node CN to the front multiplex node FN in accordance with the frame format shown in FIG. 5(c). In the model B, the signals are produced by different multiplex nodes, and therefore, they are transmitted by means of two different frames having their data areas assigned with the respective signals.
With conventional frame formats, however, signal allocation in the data area must be changed depending on the model and grade of vehicles, and therefore, the individual frame formats must be modified. That is, depending on the model and grade of vehicles, the assignment of all the signals to the frame ID and the data area must be determined, thus causing an increase in load of development of multiplex transmission systems and in cost.